Bring documentation closer to built-in editor

### Godot version

4.4.1.stable

### VS Code version

1.99.3

### Godot Tools VS Code extension version

2.5.1

### System information

Windows 11

### Problem statement

Currently, the built-in GDScript editor can show all of the documentation flawlessly. This includes all of the built-in documentation, but also custom documentation added with `##` documentation comments. This doesn't work the same way in VS Code, as many features are missing.

### Proposed solution

- add full support for BBCode-like tags and class reference. It would be nice to be able to click on any references and follow them like in the built-in editor, as well as have them render properly. Currently, using `[param argument_name]` makes it render as `param argument_name` instead of just `argument_name`; this is inconsistent, as some tags render properly (like `[signal signal_name]`)
- render line breaks when there's an empty line with `##`. The following currently renders all on the same line in VS Code:
```
## First line.
##
## Should be on the next line.
```
- fix custom `class_name` documentation comments, as they're currently not showing up at all